Optical nonlinearity in organic and organometallic molecules via lattice inclusion complexation

ABSTRACT

There are disclosed a nonlinear optical element capable of second harmonic generation comprising a crystalline inclusion complex of a lattice-forming host compound crystallized with continuous channel cavities in the presence of a nonlinearly polarizable guest compound in a noncentrosymmetric space group, said guest compound having specified properties, and both said guest and host being selected from specified classes; a nonlinear optical device; a method of generating second harmonic radiation; and an electro-optic modulator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to nonlinear optical systems, and particularly toorganic and organometallic complexes capable of second harmonicgeneration (SHG) and having other useful nonlinear optical andelectro-optic properties.

2. Description of Related Art

Nonlinear second order optical properties, such as second harmonicgeneration and the linear electrooptic effect, arise from the firstnonlinear term, χ.sup.(2) EE, in the dipolar approximation of thepolarization induced in a material by an applied electric field, E.

P(induced)=χ.sup.(1) E+χ.sup.(2) EE+χ.sup.(3) EEE+. . . The vectorquantities P and E are related by tensor quantities X.sup.(1),X.sup.(2), X.sup.(3) . . . , where X.sup.(1) is the linearsusceptibility, X.sup.(2) is the second order susceptibility whicharises from the second order molecular hyperpolarizability, X.sup.(3) isthe third order susceptibility which arises from furtherhyperpolarizabilities, etc. As tensor quantities, the susceptibilities,X.sup.(i), are highly symmetry dependent; odd order coefficients arenonvanishing for all materials but even order coefficients, e.g.,χ.sup.(2) which is responsible for SHG, are nonvanishing only fornoncentrosymmetric materials.

Franken, et al., Physical Review Letters, 7, 118-119 (1961), disclosethe observation of second harmonic generation (SHG) upon the projectionof a pulsed ruby laser beam through crystalline quartz. The use of alaser remains the only practical way to generate an E large enough to beable to detect the SHG phenomenon.

Although a large number of organic and inorganic materials capable ofSHG have been found, an intensive search continues for molecules whichexhibit large hyperpolarizabilities, β. An organic molecule having aconjugated π-electron system or a low-lying charge transfer excitedstate often has an extremely large molecular hyperpolarizability, butunfavorable alignment in the crystalline phase can result in acentrosymmetric material in which χ.sup.(2) vanishes. It is possible tocircumvent this problem by using a chiral molecule to insure arigorously noncentrosymmetric crystal, but problems associated with thecreation and maintenance of a high level of optical purity limit thevalue of this approach. Moreover, optical activity does not guaranteethat X.sup.(2) will be large, only that it will be nonzero.

Tomaru, et al., J. Chem. Soc. Chemical Communications, 1207-1208 (1984),disclose SHG in inclusion complexes between dimethyl β-cyclodextrin as ahost molecule and a guest molecule chosen from p-nitroaniline,2-hydroxy-4-nitroaniline and N-methyl-4-nitroaniline. The guests have acrystalline centrosymmetric geometry which is removed by formation ofthe inclusion complex.

Wang, et al., Chemical Physics Letters, 120, 441-444 (1985), discloseSHG with a crystalline 1:1 inclusion complex between p-nitroaniline andβ-cyclodextrin (CD) as a host when exposed to the 1.06 μm output of aNd-YAG laser. The authors also disclose SHG for CD inclusion complexesof other guests, specifically p-(N,N-dimethylamino)cinnamaldehyde,N-methyl-p-nitroaniline, 2-amino-5-nitropyridine andp-(dimethylamino)benzonitrile.

Cyclic inclusion complexes are those in which the host is a macrocyclicmolecule characterized by a relatively large diameter hole in thecenter. Such compounds are known to include smaller molecules inside thecavity created by the interior void. A much larger class of inclusioncomplexes is the lattice inclusion complexes, in which the hostco-crystallizes with the other material (guest) included within thelattice structure. These complexes are distinct from the cyclicinclusion complexes because for the former the region of the finalcrystalline structure in which the guest is located is defined by thevoids created by the arrangement of host atoms in the unit cell of thehost structure.

A useful review of the art relating to nonlinear properties of organicmaterials is given in Nonlinear Optical Properties of Organic andPolymeric Materials, D. J. Williams, ed., American Chemical Society,Washington, D.C. (1983). The structures, physical properties andapplications of known inclusion compounds are reviewed in InclusionCompounds, Atwood, et al., eds., Academic Press, London (1984). Thesepublications do not disclose utility of lattice inclusion compounds forSHG.

Nesmeyanov, et al., Doklady Chem., 221, 229-231 (1975), translated fromDoklady Akademii Nauk SSSR, 221, 624-626 (1975), disclose the separationof metallocenes by inclusion compounds with thiourea. Specificallymentioned are the adducts with ferrocene, nickelocene, andcyclopentadienyltricarbonylmanganese.

Clement, et al., J. Chem. Soc. Chemical Communications, 654-655 (1974),disclose that ferrocene or mixtures of ferrocene and nickelocene formclathrates with thiourea. They report for the ferrocene adduct amolecular ratio of thiourea to ferrocene of 3.0 to 1. Bozak, et al.,Chemistry Letters, 75-76 (1975), disclose the incorporation ofcyclopentadienylmanganese tricarbonyl (cymantrene) into a ferroceneclathrate of thiourea when the ferrocene to cymantrene weight ratio isabout 4:1.

The search continues for other useful material for SHG.

SUMMARY OF THE INVENTION

The present invention provides a nonlinear optical element capable ofsecond harmonic generation, comprising a crystalline inclusion complexof a lattice-forming host compound crystallized with continuous channelcavities in the presence of a nonlinearly polarizable guest compound ina noncentrosymmetric space group, said guest compound

(i) being nonlinearly polarizable in the presence of an electromagneticfield, and

(ii) having a molecular width, W, such that D/2<W<D, where D is thediameter of the channel cavity, wherein

(A) the lattice-forming host compound is selected from

(a) Hofmann clathrate lattice compounds having the formula M¹ (NH₃)₂Ni(CN)₄ wherein M¹ is Mn, Ni, Cu or Cd;

(b) Werner coordination complexes of a specified formula;

(c) cyclophosphazenes;

(d) tris-ortho-thymotide;

(e) urea, thiourea and selenourea;

(f) phenols, hydroquinones and Dianin's compound;

(g) perhydrotriphenylene;

(h) cyclotriveratrylene;

(i) trianthranilides; and

(j) deoxycholic acid; and

(B) the guest compound is selected from the group consisting of certainsubstituted aromatic compounds and octahedrally coordinated transitionmetal complexes having a -bonded ligand and a specified formula with theprovisos that

said inclusion complex is other than a complex of thiourea and benzenemolybdenum tricarbonyl or a complex of tris-ortho-thymotide and stilbenechromium tricarbonyl, and

when said nonlinear optical element is a single crystal, it is otherthan an inclusion complex of thiourea and cyclopentadienylmanganesetricarbonyl. The invention also provides a nonlinear optical device andan electro-optic modulator using as an optical element the nonlinearoptical element of the invention, and a method of generating secondharmonic radiation using said nonlinear optical element. In the device,method and modulator of the invention the optical element can also be asingle crystal of an inclusion complex of thiourea andcyclopentadienylmanganese tricarbonyl.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plan view of a nonlinear optical device according to theinvention.

FIG. 2 is a plan view of an electro-optic modulator of the invention.

DETAILED DESCRIPTION OF THE INVENTION

It has been found that lattice inclusion complexation can be used toinduce optical nonlinearity in guest molecules whose crystals would notnormally exhibit such nonlinearity because the crystals arecentrosymmetric. The nonlinear optical element of the present inventioncomprises a host-guest inclusion complex of a particular lattice-forminghost compound and a nonlinearly polarizable guest compound. As usedherein the expression "lattice-forming host compound" means a compoundwhich crystallizes in a crystal structure such that channel-likecavities are formed by the framework.

In the present invention the host-guest inclusion complex is a compoundwhich crystallizes with continuous channel cavities in the presence of anonlinearly polarizable guest compound in a noncentrosymmetric spacegroup. Organometallic and inorganic lattice-forming host compounds whichare suitable for the present invention are selected from:

(a) lattice compounds of the Hofmann-type (Hofmann) clathrates, M¹(NH₃)₂ Ni(CN)₄ wherein M¹ is Mn, Ni, Cu or Cd;

(b) Werner coordination complexes having the formula M² X₂ A₄ wherein M²is divalent and is Fe, Co, Ni, Cu, Zn, Cd, Mn, Hg or Cr; X is NCS⁻,NCO⁻, CN⁻, NO₃ ⁻, NO₂ ⁻, Cl⁻, Br⁻, or I⁻ ; and A is substitutedpyridine, α-arylalkylamine or isoquinoline;

(c) cyclophosphazenes. When Werner coordination complexes are used,suitable substituents for pyridine include hydrogen, linear or branchedC₁ -C₆ alkyl, C₃ -C₇ cycloalkyl or phenyl groups, halogen, C₁ -C₆alkylamine groups and nitro groups. Suitable alkyl groups of thearylalkylamine are linear or branched primary C₁ -C₆ alkyls. Suitablearyl groups of the arylalkylamine include phenyl and substituted phenyl,in which the substituents can be linear C₁ -C₆ or branched C₃ -C₆ alkyl,C₃ -C₇ cycloalkyl, phenyl, halogen, C₁ -C₆ alkylamino or nitro.

Organic lattice-forming host compounds suitable for use in the presentinvention are selected from

(a) tris-ortho-thymotide;

(b) urea, thiourea and selenourea;

(c) phenols, hydroquinones and Dianin's compound;

(d) perhydrotriphenylene;

(e) cyclotriveratrylene;

(f) trianthranilides; and

(g) deoxycholic acid.

Preferably, the lattice-forming host compound is tris-ortho-thymotide(TOT), a trianthranilide, cycloveratrylene, deoxycholic acid, urea,thiourea or selenourea; and most preferably, TOT, thiourea o deoxycholicacid.

For a given lattice-forming host compound set forth above, there is arange of sizes and shapes of guest molecules which can be accommodated.Some lattice-forming host compounds, e.g., urea, provide rigid channelswith a narrow range of acceptable guest sizes. Other lattice-forminghost compounds, e.g., the trianthranilides, TOT and theperhydrotriphenylenes, either have channels of different sizes in thesame crystal structure or frameworks capable of expanding or contractingto incorporate a wider range of guest sizes. The approximate dimensionsof the channels for many of the lattice-forming host compounds suitablefor use in the present invention are given in Inclusion Compounds,Atwood, et al., eds., Academic Press, London (1984). This disclosure isincorporated herein by reference. Observed ranges of the width ofchannel cavities in the foregoing lattice-forming hosts whencrystallized with a guest are given in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        HOST              WIDTH (Å)                                               ______________________________________                                        Hofmann compounds 7-9                                                         Werner complexes  7-9                                                         Cyclophosphazenes 5-10                                                        Hydroquinones,    2.6-3 (spherical radius)                                    Dianin's compound 6                                                           Urea              5.2                                                         Thiourea          6.1                                                         Perhydrotriphenylene                                                                            ˜5                                                    Cyclotriveratrylene                                                                             ˜5                                                    Trianthranilides  5.5-9                                                       Deoxycholic acid  4-6                                                         TOT               4-6                                                         ______________________________________                                    

It is to be understood that depending upon the guest the channel sizescan expand or contract slightly. As used herein the width or diameter ofthe channel is defined as the average of the diameters of the largestcylinder which can be accommodated within the channel and the smallestcylinder which can enclose the void space of the channel.

In the present invention, for a given channel size, the guest moleculeis nonlinearly polarizable in the presence of an electromagnetic fieldand has a molecular width, W, such that D/2<W<D, where D is the diameterof the channel. This geometric property insures that the guest moleculesare small enough to fit into the channels of the lattice-forming hostcompound, but large enough to prevent the formation of pairedhead-to-tail dimers, which would be incapable of SHG. As used herein,"molecular width" and "molecular length" of a guest molecule can beestimated by building a molecular model of the molecule usingcommercially available kits, e.g. CPK space-filling or Dreiding stickmodels, and measuring the longest dimension (L) and the width (W) of thecylinder obtained by turning the model about the axis formed by L. Themeasurements are performed with a scale ruler calibrated according tothe kit used. The dimensions of potential guest molecules can also beobtained from X-ray diffraction crystal structure data.

Guest molecules suitable for use in the present invention have anon-zero molecular second order polarizability (β). Molecules whichexhibit a non-zero dipole moment change between the ground state andsome of their excited states possess a non-zero molecular second orderpolarizabilities. Normal values of β, for materials which are linear intheir response are about 10⁻³⁰ esu (electrostatic units), whereas highlypolarizable materials have a β value of about 10⁻²⁷ esu. In the presentinvention the guest molecule has second order polarizability greaterthan 10⁻³⁰ esu.

In general, the molecular interaction by which the host-guestcomplexation is generated should exhibit directional selectivity forboth guest and host to minimize orientational cancellation of bulksecond order optical properties. Spectroscopic measurements can beemployed to determine in each case whether anisotropic host-guestcomplexation occurs.

Guest compounds suitable for preparing a crystalline inclusion complexfor nonlinear optical elements of the invention are selected from thegroup consisting of

(a) substituted aromatic compounds of the formula ##STR1## wherein A isC or N;

R¹ is --NH₂, --NHCH₃, --N(CH₃)₂, or --C(O)M(CO)_(x) where M is Mn or Reand x is 5 or M is Co and x is 4;

R² is --NO₂, --CN, --(CH═CH)_(n) C(O)H where n is 1 to 3, or4-(dicyanomethylene)-2-methyl-6-vinyl-4H-pyran; and

Y is --H, --CH₃, --OCH₃, --OH, --F or Cl; and

(b) octahedrally coordinated transition metal complexes having a -bondedligand and having the formula

    [L.sup.1 M.sup.3 L.sup.2.sub.m ].sup.p

wherein

M³ is Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Rh, or Ir;

L¹ is an olefinic or aromatic ligand capable of -bonding to M³ to formpart of an octahedral complex;

each L² is independently an unidentate ligand;

m is 1-3 and is the total number of unidentate ligands, L² ; and

p is --1, 0 or +1; with the proviso that said inclusion complex is otherthan a complex of thiourea and benzene molybdenum tricarbonyl or acomplex of tris-ortho-thymotide and stilbene chromium tricarbonyl.Single crystals of an inclusion complex of thiourea andcyclopentadienylmanganese tricarbonyl are not included within the scopeof the nonlinear optical elements of the invention but are within theother embodiments of the invention. When p is ±1, the transition metalcomplex is associated with appropriate counter ions. Suitable anionsinclude BF₄ ⁻, PF₆ ⁻, halides, ClO₄ ⁻, NO₃ ⁻ and SbF₆ ⁻. Suitablecations include alkali metal ions, N(CH₃)₄ ⁺, tetraethyl ammonium ion,and tetramethyl phosphonium ion.

Preferably, M³ is Cr, Re, Fe or Mn. Preferably, L¹ is selected from thegroup consisting of substituted and unsubstituted benzenes, naphthalene,indene, substituted and unsubstituted cyclic and acyclic 1,3 dienes,cyclopentadiene and substituted cyclopentadienes, and trimethylenemethane. Preferably, L² is --CO, --CS, --SCN, --NO, or --CN; mostpreferably --CO. When the guest compound is an octahedrally coordinatedtransition metal complex, the lattice forming host compound ispreferably TOT or thiourea.

When the guest compound is a substituted aromatic compound as specifiedabove, preferred embodiments of the invention are R¹ is --N(CH₃)₂ or--C(O)Mn(CO)₅, R² is --CN, --CH═CHC(O)H or4-(dicyanomethylene)-2-methyl-6-vinyl-4H-pyran, and the lattice hostcompound is TOT or deoxycholic acid. Preferably A is C and Y is H.

The inclusion complexes used in the present invention can be prepared bya process comprising adding the guest compound, optionally dissolved ina suitable solvent, to a solution of the lattice-forming host compounddissolved in a suitable solvent, mixing the resulting combination,heating if necessary to effect complete dissolution, filtering theresulting solution and forming crystals of the desired product bycooling the filtered solution or slowly evaporating the solvent.Suitable solvents include water and volatile organic solvents such asalcohols, ethers, aromatic hydrocarbons and chlorinated hydrocarbons.The actual solvent used will depend upon the guest and host compounds.Methanol and ethanol are preferred solvents. Dissolution of the host andguest compound is effected at a temperature of from about 20° C. toabout 100° C., preferably about 20° C. to about 65° C. To effectcrystallization of the inclusion complex, the filtered solution isgenerally cooled to about -30° C. to about 0° C.

The nonlinear optical device of the invention comprises means to directat least one incident beam of electromagnetic radiation into an opticalelement having nonlinear optical properties whereby electromagneticradiation emerging from said element contains at least one frequencydifferent from the frequency of any incident beam of radiation, saiddifferent frequency being an even multiple of the frequency of oneincident beam of electromagnetic radiation; said optical elementcomprising a crystalline inclusion complex of a lattice-forming hostcompound crystallized with continuous channel cavities in the presenceof a nonlinearly polarizable compound in a noncentrosymmetric spacegroup, said guest compound

(i) being nonlinearly polarizable in the presence of an electromagneticfield, and

(ii) having a molecular width, W, such that D/2<W<D, where D is thediameter of the channel cavity; wherein the lattice-forming hostcompound and guest compound are selected as set forth earlier herein forthe nonlinear optical element of the invention, but the inclusioncomplex can also be a single crystal of an inclusion complex of thioureaand cyclopentadienylmanganese tricarbonyl.

Preferably, the emerging radiation of a different frequency is doubled(second order) (SHG). Preferably, the electromagnetic radiation isradiation from one of a number of common lasers, such as Nd-YAG,semiconductor diode, and Ar or Kr ion. Referring now to FIG. 1, opticalelement 1 is oriented in one of a potentially infinite number of crystalorientations which achieve partially maximized SHG conversion by virtueof phase matching. The specific orientation is chosen for reasons ofnoncriticality, maximum nonlinearity, increased angular acceptance, etc.Polarized light of wavelength 1.06 μ from Nd-YAG laser 2 is incident onthe optical element along the optical path. A lens 3 focuses the lightinto the optical element. Light emerging from the optical element 1 iscollimated by a similar lens 4 and passed through a filter 5 adapted toremove light of wavelength 1.06 μ while passing light of wavelength 0.53μ.

Optical element 1 is preferably a single crystal having at least onedimension of about 0.5 mm or greater but can be substantially smallercrystals imbedded in a film of polymer. The smaller crystals can berandomly orientated or aligned with the same orientation, and arepreferably aligned. For the smaller crystals, if their size is smallenough to prevent light scattering, they can be dispersed in thepolymeric binder and pressed, molded or shaped into an optically clearelement capable of SHG. The polymer binder should be chosen to be anon-solvent for the inclusion complex. For larger crystallites, similarelements can be prepared if the binder used has an index of refractionmatched to the complex, so as to prevent light scatter and remaintransparent.

It will be further apparent to those skilled in the art that the opticalelements of the invention are useful in other devices utilizing theirnonlinear properties, such as sum and difference frequency mixing,parametric oscillation and amplification, and the electro-optic effect.The use of crystals having nonlinear optical properties in opticaldevices is known in the art, as shown by U.S. Pat. Nos. 3,747,022,3,328,723, 3,262,058 and 3,949,323.

The electro-optic modulator of the invention comprises means to direct acoherent beam into an optical element, and means to apply an electricfield to said element in a direction to modify transmission property ofsaid beam, said optical element meeting the description given above forthe optical element for the nonlinear optical device of the invention.The preferred optical elements for the nonlinear optical device andelectro-optic modulator of the invention are those embodiments set forthearlier herein for the nonlinear optical element, and also a singlecrystal of an inclusion complex of thiourea andcyclopentadienylmanagnese tricarbonyl.

Referring now to FIG. 2, an electro-optic modulator embodying theinvention utilizes optical element 11. A pair of electrodes 12 and 13are attached to the upper and lower surfaces of the element 11, acrosswhich a modulating electric field is applied from a conventional voltagesource 14. Optical element 11 is placed between polarizers 15 and 17. Alight beam 18, such as that from as Nd-YAG laser, is polarized bypolarizer 15, focused on the optical element 11, propagated through thecrystal or crystals and subjected to modulation by the electric field.The modulated light beam is led out through analyzer polarizer 17.Linearly polarized light traversing element 11 is rendered ellipticallypolarized by action of the applied modulating voltage. Polarizer 17renders the polarization linear again. Application of the modulatingvoltage alters the birefringence of element 11 and consequently theellipticity impressed on the beam. Polarizer 17 then passes a greater orlesser fraction of the light beam as more or less of the ellipticallypolarized light projects onto its nonblocking polarization direction.

It is understood that the invention has been described with reference topreferred embodiments thereof and that variations are to be includedwithin the scope of the invention. Furthermore, frequency or phasemodulation of the light beam by the modulator is possible, although theembodiment specifically described performs intensity modulation.

The invention is further illustrated by the following examples in whichall percentages and parts are by weight and temperatures are in degreesCesius, unless otherwise stated. All of the complexes illustrated in theExamples have a guest with size requirements and polarizabilities as setforth herein, and the complexes crystallize in noncentrosymmetric spacegroups with channel cavities.

Experimental Procedure for Measuring SHG

Unless otherwise stated SHG was measured by the powder method of Kurtz,et al., J. Appl. Phys. 39, 3798 (1968), using a Nd YAG laser (λ=1.064μm) and urea as a reference. The polycrystalline urea powder used as areference had an average particle size of 90 μm to 125 μm. The intensityof the second harmonic radiation generated by each sample tested wasthus measured relative to that provided by urea.

General Procedures

The following preparative procedures were used in the Examples asindicated:

A. About 200-500 mg of a selected organometallic complex were added toabout 10-20 mL of a 50% saturated solution of thiourea in methanol. Theresulting mixture was heated if necessary to dissolve the organometalliccomplex; and the resulting solution was filtered under pressure. Thefiltered solution was cooled to about 0° to -20° whereby crystalsformed.

B. Thiourea and the selected organometallic complex in a molar ratio ofabout 3:1 were dissolved in methanol and then the methanol wasevaporated under reduced pressure causing formation of solid. Theresulting solid product isolated from this procedure provided materialsuitable for SHG measurements but not for X-ray diffraction analyses.

C. A mixture of about 150 mg of TOT in about 50 mL of methanol washeated until the TOT dissolved, and the resulting solution was cooled toambient temperature, about 25°. About 150 mg of the selected guestcompound were added to the TOT/methanol solution, and the resultingmixture was heated if necessary to dissolve the guest compound. Theresulting solution was filtered under pressure and then the methanol wasevaporated slowly at ambient temperature.

EXAMPLE 1

Thiourea (4 g) was added to a solution of 0.5 g of benzene chromiumtricarbonyl in 50 mL of methanol in a round-bottom flask, and theresulting mixture was heated at about 30° and swirled until the thioureadissolved. Then the methanol was evaporated under reduced pressure tocause the formation of a crystalline product, which was removed from theflask and washed with petroleum ether until the washings were colorlessto give 4.47 g of light yellow needles as product.

Procedure A was used with 15 mL of thiourea solution and 250 mg ofbenzene chromium tricarbonyl. After keeping the resulting solution ofproduct at 0° for two days, yellow needles were isolated: mp=158°-170°(decomp.). IR(KBr): 1948s, 1892m, 1879s, 1848w, 1632w, 1615m, 1490wcm⁻¹.

Another sample of the thiourea/benzene chromium tricarbonyl inclusioncomplex was prepared according to Procedure A as described above. Anal:Calcd. for C₁₂ H₁₈ CrN₆ O₃ S₃ : C, 32.57; H, 4.10; Cr, 11.75. Found: C,32.72; H 4.18; Cr, 11.29. Results of SHG measurements on the materialsprepared in this Example are presented in Table 2.

EXAMPLE 2

Procedure A was used with 15 mL of thiourea solution and 250 mg offluorobenzene chromium tricarbonyl. The chromium complex was dissolvedwithout heating; and then the resulting solution was cooled first to -1°and next to about -20° to give yellow needles of the inclusion complexwhich were separated by hand from crystals of thiourea. IR(KBr): 1958vs,1932w, 1892vs, 1632s, 1489m, 1461m cm⁻¹. Anal.: Calcd. for C₁₂ H₁₇ CrFN₆O₃ S₃ : C, 31.30; H, 3.72; Cr, 11.29. Found: C, 31.13; H, 3.88; Cr,11.13. SHG results are presented in Table 2.

EXAMPLE 3

Procedure B was used with 100 mg of cyclopentadienylrhenium tricarbonyland 100 mg of thiourea in 15 mL of methanol. SHG results are presentedin Table 2.

EXAMPLE 4

Procedure A was used with 250 mg of 1,3-cyclohexadiene iron tricarbonyland 15 mL of thiourea solution. Light yellow needles were isolated asproduct. IR(KBr): 2043s, 1964s, 1957sh, 1613s cm⁻¹. Anal.: Calcd. forC₁₂ H₂₀ FeN₆ O₃ S₃ : C, 32.15; H, 4.50; Fe, 12.46. Found: C, 32.01; H,4.47; Fe, 12.60. SHG results are presented in Table 2.

EXAMPLE 5

Procedure A was used with 400 mg of cyclohexadienylmanganese tricarbonyland 15 mL of thiourea solution. After maintaining the resulting solutionof product at -20° for two days, crystals were isolated. IR(KBr): 2006s,1945sh, 1931s, 1919s, 1633w, 1615s cm⁻¹. Anal Calcd. for C₁₂ H₁₉ MnN₆ O₃S₃ : C, 32.28; H, 4.29; Mn, 12.31. Found: C, 32.11; H, 4.43; Mn, 12.68.

Procedure B was also used to prepare the inclusion complex from 50 mg ofcyclohexadienylmanganese tricarbonyl and 50 mg of thiourea in 10 mL ofmethanol. SHG results are presented in Table 2.

EXAMPLE 6

Procedure A was used with 300 mg of trimethylenemethane iron tricarbonyland 15 mL of thiourea solution. The resulting solution was cooled to-18° overnight to yield long, thin, off-white needles of the inclusioncomplex which were isolated. Another sample of the complex was preparedusing a similar procedure. IR(KBr): 2060m, 1986s, 1613s cm⁻¹. Anal :Calcd. for C₁₀ H₁₈ FeN₆ O₃ S₃ : C, 28.44; H, 4.30; Fe, 13.22. Found: C,28.36; H, 4.33; Fe, 12.79. SHG results are presented in Table 2.

EXAMPLE 7

Thiourea (4.0 g) was added to a solution of 0.5 g ofcyclopentadienylmanganese tricarbonyl dissolved in 50 mL of methanol.The resulting mixture was heated to 30° to dissolve the thiourea; themethanol was removed under reduced pressure to give solid product whichwas washed with petroleum ether to yield 4.42 g of pale yellow crystals.SHG results are presented in Table 2.

This inclusion complex was also prepared using Procedure A with 500 mgof cyclopentadienylmanganese tricarbonyl and 10 mL of thiourea solution.The resulting solution was maintained at 0° for about 4 days to give 230mg of crystals which were washed with methanol and dried under nitrogen.IR(KBr): 2012s, 2006sh, 1954w, 1928vs, 1634w, 1616s, 1490w cm⁻¹. AnalCalcd. for C₁₁ H₁₇ MnN₆ O₃ S₃ : C, 30.55; H, 3.96; Mn, 12.70. Found: C,30.74; H, 4.25; Mn, 10.80.

EXAMPLE 8

Procedure A was used with 400 mg of butadieneiron tricarbonyl and 10 mLof thiourea solution. The resulting solution was maintained at -15° fortwo days to give crystals which were isolated, washed with methanol anddried under n to yield 129 mg of product. SHG results are presented inTable 2. The material turned white when left standing for a period of atleast a week, thereby suggesting that the organometallic compound may becoming out of the inclusion complex.

EXAMPLE 9

Procedure B was used with 100 mg of pyrroylmanganese tricarbonyl and 100mg of thiourea in 15 mL of methanol to give a yellow solution from whichthe inclusion complex was isolated. SHG results are given in Table 2.

EXAMPLE 10

Procedure A was used with 400 mg of cyclopentadienylchromium dicarbonylnitrosyl and 15 mL of thiourea solution. The resulting solution wasmaintained at 0° for two weeks to yield fiber-like needles which wereisolated. IR(KBr): 2019sh, 2010.88s, 1941m, 1700m, 1615s cm⁻¹. Anal.:Calcd. for C₁₀ H₁₇ CrN₇ O₃ S₃ : C, 27.84; H, 3.97; Cr, 12.05. Found: C,27.80; H, 3.95; Cr, 12.30. SHG results are given in Table 2.

EXAMPLE 11

TOT (150 mg) was dissolved in 50 mL of boiling methanol. The resultingsolution was cooled to ambient temperature, about 25°, decanted into asolution of 200 mg of p-dimethylaminocinnamaldehyde in 10 mL ofmethanol. The resulting product solution was maintained at 0° for 6 daysto give yellow rhomboid crystals which were isolated to yield 90 mg ofproduct (mp=217°-218°). Procedure C was used with 200 mg ofp-dimethylaminocinnamaldehyde. Solvent was evaporated from the resultingsolution to give the product. The ratio of host to guest was determinedby X-ray crystallography. Anal.: Calcd. for C₇₇ H₈₅ NO₁₃ : C, 75.03; H,6.95. Found: C, 74.81; H, 6.82. SHG results are given in Table 3.

EXAMPLE 12

Procedure C was used with 150 mg of dimethylaminobenzonitrile . Solventwas evaporated from the resulting solution to give white crystals whichwere isolated to yield 90 mg of product. Duplicate elemental analysesfor C (74.91, 74.66) and H (6.56, 6.56) are consistent with both a hostguest ratio of 1:1 calculated for C₄₂ H₄₆ N₂ O₆ (C, 74.75; H, 6.87) and2:1 calculated for C₇₅ H₈₂ O₁₂ N₂ (C, 74.88; H, 6.87). SHG results aregiven in Table 3.

EXAMPLE 13

TOT (50 mg) was dissolved in 15 mL of boiling methanol to give asolution which was cooled to ambient temperature and then filtered intoa flask containing 100 mg of p-cyanobenzoyl manganese pentacarbonyl.From the resulting solution yellow needles, which probably contained anexcess of the organometallic complex, were isolated.

In a separate experiment, using Procedure C with 150 mg of TOT and 150mg of the organometallic complex, thin white needles were isolated. SHGresults are presented in Table 3.

EXAMPLE 14

Procedure C was used with 250 mg of indanechromium tricarbonyl to yield157 mg of product. IR(KBr): 1958s, 1874s, 1765m, 1269w, 1257w, 1219m,1092m. Anal.: Calcd. for C₄₅ H₄₆ CrO₉ : C, 69.04 H; 5.92. Found: C,68.97; H, 5.71. SHG results are presented in Table 3.

EXAMPLE 15

A solution of 50 mg of TOT in 15 mL of methanol was added to 58 mg ofmethoxybenzene chromium tricarbonyl to give a solution from which themethanol was evaporated to obtain a light yellow crystalline product.

In a separate experiment, Procedure C was used with 250 mg of theorganometallic complex to give 62 mg of the same product. SHG resultsare given in Table 3.

EXAMPLE 16

Procedure C was used with 300 mg of 1,2,3,4-tetrahydronaphthalenechromium tricarbonyl. Some solvent was evaporated from the resultingsolution to give yellow needles. SHG results are given in Table 3.

EXAMPLE 17

Procedure C was used with 300 mg of (benzenemanganesetricarbonyl)tetrafluoroborate. Some solvent was evaporated from theresulting solution to give needles of the organometallic startingmaterial and block-like crystals of the inclusion complex. SHG resultsare given in Table 3.

                  TABLE 2                                                         ______________________________________                                        Inclusion Complexes of Thiourea                                                             Host:Guest                                                                              SHG (Rel.                                             Example       Ratio     to urea)                                              ______________________________________                                        1             3:1        0.8-2.25                                             2             3:1       2                                                     3             (a)       0.5                                                   4             3:1       0.4                                                   5             3:1       0.4                                                   6             3:1       0.3                                                   7             3:1       0.25-0.3                                              8             (a)       1.0                                                   9             (a)       0.2                                                   10            3:1       0.1                                                   ______________________________________                                         (a) No elemental analysis was obtained                                   

                  TABLE 3                                                         ______________________________________                                        Inclusion Complexes of Tris-ortho-thymotide                                                 Host:Guest                                                                              SHG (Rel.                                             Example       Ratio     to urea)                                              ______________________________________                                        11            2:1 (b)   0.6-1.0                                               12            (c)       0.25                                                  13            (a)       0.2                                                   14            1:1       0.1                                                   15            (a)       0.1                                                   16            1:1       0.1                                                   17            (a) (b)   0.1                                                   ______________________________________                                         (a) Elemental analysis was not obtained.                                      (b) Host:guest ratio determined by Xray diffraction.                          (c) Host:guest ratio cannot be determined by C,H analysis.               

EXAMPLE 18

Deoxycholic acid (400 mg) and 200 mg of p-nitroaniline were dissolved in40 mL of ethanol. The resulting solution was filtered under pressure andthe solvent evaporated. The resulting solid was removed by filtration,rinsed with cold methanol (2 mL) and dried under reduced pressure togive 365 mg of a yellow product. The product gave a SHG of 1.0 relativeto urea.

EXAMPLE 19

Deoxycholic acid (1 g) in 5 mL of heated ethanol was added to 0.1 g of4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran in 5 mLof heated ethanol. The resulting mixture was filtered and the filtratewas allowed to cool to ambient temperature. After the filtered solutionwas maintained at ambient temperature overnight, it was filtered underpressure, and then the solvent was evaporated to give the product whichdisplayed a SHG of 0.13-0.38 relative to urea.

The Invention Being Claimed Is:
 1. A nonlinear optical element capableof second harmonic generation, comprising a crystalline inclusioncomplex of a lattice-forming host compound crystallized with continuouschannel cavities in the presence of a nonlinearly polarizable guestcompound in a noncentrosymmetric space group, said guest compound(i)being nonlinearly polarizable in the presence of an electromagneticfield, and (ii) having a molecular width, W, such that D/2<W<D, where Dis the diameter of the channel cavity;wherein (A) the lattice-forminghost compound is selected from the group consisting of(a) Hofmannclathrate lattice compounds having the formula M¹ (NH₃)₂ Ni(CN)₄ whereinM¹ is Mn, Ni, Cu or Cd; (b) Werner coordination complexes having theformula M² X₂ A₄ wherein M² is divalent and is Fe, Co, Ni, Cu, Zn, Cd,Mn, Hg or Cr; X is NCS⁻, NCO⁻, CN⁻, NO₃ ⁻, NO₂ ⁻, Cl³¹ , Br⁻, or I⁻ ;and A is substituted pyridine, α-arylalkylamine or isoquinoline; (c)cyclophosphazenes; (d) tris-ortho-thymotide; (e) urea, thiourea andselenourea; (f) phenols, hydroquinones and Dianin's compound; (g)perhydrotriphenylene; (h) cyclotriveratrylene; (i) trianthranilides; and(j) deoxycholic acid; and (B) the guest compound is selected from thegroup consisting of(a) substituted aromatic compounds of the formula##STR2## wherein A is C or N;R¹ is --NH₂, --NHCH₃, --N(CH₃)₂, or--C(O)M(CO)_(x) where M is Mn or Re and x is 5 or M is Co and x is 4; R²is --NO₂, --CN, (CH═CH)_(n) C(O)H where n is 1 to 3, or4-(dicyanomethylene)-2-methyl-6-vinyl-4H-pyran; and Y is --H, --CH₃,--OCH₃, --OH, --F or Cl; and (b) octahedrally coordinated transitionmetal complexes having a -bonded ligand and having the formula

    [L.sup.1 M.sup.3 L.sup.2.sub.m ].sup.p

whereinM³ is Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Rh or Ir; L¹ is anolefinic or aromatic ligand capable of -bonding to M³ to form a part ofan octahedral complex; each L² is independently an unidentate ligand; mis 1-3; and p is -1, 0 or +1; with the provisos that said inclusioncomplex is other than a complex of thiourea and benzene molybdenumtricarbonyl or a complex of tris-ortho-thymotide and stilbene chromiumtricarbonyl, and when said nonlinear optical element is a singlecrystal, it is other than an inclusion complex of thiourea andcyclopentadienylmanganese tricarbonyl.
 2. A nonlinear optical elementaccording to claim 1 wherein the lattice-forming host compound isselected from the group consisting of(a) tris-ortho-thymotide; (b) urea,thiourea and selenourea; (c) phenols, hydroquinones and Dianin'scompound; (d) perhydrotriphenylene; (e) cyclotriveratrylene; (f)trianthranilides; and (g) deoxycholic acid.
 3. A nonlinear opticalelement according to claim 2 wherein the guest compound is anoctahedrally coordinated transition metal complex having a -bondedligand.
 4. A nonlinear optical element according to claim 3 wherein thehost compound is tris-ortho-thymotide, a trianthranilide,cycloveratrylene, deoxycholic acid, urea, thiourea or seleonurea.
 5. Anonlinear optical element according to claim 4 wherein L¹ is selectedfrom the group consisting of substituted and unsubstituted benzenes,naphthalene, indene, substituted and unsubstituted cyclic and acylic 1,3dienes, cyclopentadiene and substituted cyclopentadienes, andtrimethylene methane.
 6. A nonlinear optical element according to claim5 wherein each L² is independently --CO, --CS, --SCN, --NO or --CN.
 7. Anonlinear optical element according to claim 6 wherein the host compoundis thiourea or tris-ortho-thymotide.
 8. A nonlinear optical elementaccording to claim 7 wherein M³ is Cr, Re, Fe or Mn.
 9. A nonlinearoptical element according to claim 8 wherein L² is CO.
 10. A nonlinearoptical element according to claim 2 wherein the guest compound is asubstituted aromatic compound.
 11. A nonlinear optical element accordingto claim 10 wherein the host compound is tris-ortho-thymotide ordeoxycholic acid.
 12. A nonlinear optical element, according to claim 11wherein A is C.
 13. A nonlinear optical element according to claim 12wherein Y is H.
 14. A nonlinear optical element according to claim 13wherein R¹ is --N(CH₃)₂.
 15. A nonlinear optical element according toclaim 14 wherein R² is --CN and the host is tris-ortho-thymotide.
 16. Anonlinear optical element according to claim 14 wherein R² is--CH═CHC(O)H and the host is tris-ortho-thymotide.
 17. A nonlinearoptical element according to claim 13 wherein R¹ is --C(O)Mn(CO)₅ andthe host is tris-ortho-thymotide.
 18. A nonlinear optical elementaccording to claim 14 wherein R² is4-(dicyanomethylene)-2-methyl-6-vinyl-4H-pyran and the host isdeoxycholic acid.